Matrix assisted laser desorption/ionization (MALDI), which provides the soft desorption/ionization of a wide range of analytes, has attracted the attention of scientists with different interests. The technique is suitable for desorption/ionization of molecules with different sizes and physical properties. Presently it is widely used for mass spectrometry of proteins, peptides, lipids, intact microorganisms, synthetic polymers and even small molecules. MALDI works based on desorption/ionization of analytes using laser irradiation. Commonly either a UV or an IR laser is used. The pulsed laser is focused on a sample spot causing desorption and ionization of the sample.
To obtain a MALDI-mass spectrum it is essential to co-crystallize the sample with a matrix. Matrices are usually organic acids with non-localized electrons that can absorb the laser energy and transfer it to the analyte of interest. The latter means that during this process both analyte and matrix ions will be generated. The matrix ions and clusters do not interfere with the high molecular weight analytes, however they are problematic with regard to analysis of small molecules (MW<500 Da). Matrix ions and clusters overlap with small molecule ions resulting in masking of the peaks corresponding to analytes of interest. To overcome this problem several techniques have been suggested to make laser desorption/ionization appropriate for analysis of small molecules such as drugs, pesticides and different groups of pollutants. For example, desorption/ionization on silicon surfaces and carbonaceous materials, which are media that can desorb/ionize small molecules without generating interferences in the low mass/charge (m/z) region. The technique is so called surface assisted laser desorption/ionization (SALDI). In addition these surfaces provide a simpler sample preparation procedure by eliminating the matrix addition step compared to the commonly used matrix assisted laser desorption mass spectrometry.
However the sensitivity of surface assisted laser desorption/ionization mass spectrometry (SALDI-MS) is relatively low and to be able to detect analytes appearing in low concentrations and dirty samples a pre-concentration/clean up step is necessary before analysis. Solid-phase extraction (SPE) is widely used for pre-concentration and clean up of samples. Graphitised carbon black (GCB) has previously been used as a solid-phase extraction media and it has been previously reported that it can also act as a surface for laser desorption/ionization of small molecules (Shariatgorji M, Amini N, Thorsen G, Crescenzi C, and Ilag L, μ-Trap for the SALDI-MS Screening of Organic Compounds Prior to LC/MS Analysis. Anal. Chem. 2008, 80, 5515-5523). In the above publication, GCB particles were placed in a small cartridge (μ-Trap) and used to solid-phase extract analytes. After sample extraction, GCB particles were removed from the cartridge using a capillary, placed on a steel plate and analysed by SALDI-MS, with the result that satisfactory MS-spectra could be obtained.
Large scale analysis of samples using SPE followed by liquid chromatography/mass spectrometry (LC/MS) has been performed in 96-well format (Williams L D, Von Tungeln L S, Beland F A and Doerge D R. Liquid chromatographic-mass spectrometric determination of the metabolism and disposition of the anti-retroviral nucleoside analogs zidovudine and lamivudine in C57BL/6N and B6C3F1 mice. J Chromatogr B Analyt Technol Biomed Life Sci. 2003 Dec. 5; 798(1):55-62. The known techniques have necessitated analysing each sample with the cumbersome LC/MS technique.